In stab assemblies comprising jaws of plug-on units for connections to bus bars in a control center, attempts have been made to improve the electrical contact and extend life of the stab assemblies in various ways. The stab connector shown in U.S. Pat. No. 3,778,748 provides a spring biased insert to urge the contact jaws together to improve the electrical connection to the bus bars. The stab assembly shown in U.S. Pat. No. 3,478,299 provides a two ply spring biased jaw arrangement including steel backup springs cradling the copper contact jaws to urge them securely against the bus bars for good electrical contact.
One of the major causes of failure of stab assemblies is relative motion between the bus bars and the jaws connected thereto which eventually weakens the connection until arcing occurs or until the contact jaws heat to the softening temperature of copper, or other conductive metal of which the contact jaws are made. The millivolt drop which corresponds to the softening temperature of copper is 120 millivolts for 190.degree. C. The vertical bus bars in electric control centers to which plug-on units may be connected are moveable vertically due to thermal expansion of the bus, the motion cycle being within the range of 0.075 inches up and down when the current carried is sufficient to produce a 50.degree. C temperature rise of the bus bars.
By providing jaws floatably mounted for reciprocating movement with the bus bars in accordance with this invention, relative motion between bus bars and jaws connected thereto is avoided thereby eliminating a major cause of stab failure.